1. Field of the Disclosure
This disclosure is related to methods for determining the depth of a drillbit and using the determined depth for controlling the operation of downhole logging tools. The method of the disclosure is applicable for use with both measurement-while-drilling (MWD) tools and wireline tools.
2. Description of the Related Art
During the drilling of a hydrocarbon wellbore, surface measurements are commonly made of the amount of drillstring conveyed into the earth as a measure of the length of the drillstring in the borehole. This length is used to estimate the measured depth (or along hole length) of a borehole. Discrepancies in the length of the borehole estimated at the surface and the actual length of the borehole can result in misalignments of logs of data measured with sensors on the drillstring. One common cause of this discrepancy is an assumption that the drillstring is inelastic and therefore does not stretch.
WO2005033473 of Aldred et al. addresses this problem using a method that corrects for depth errors in drillstring measurements using a correction based on stress in the drillstring. U.S. Pat. No. 5,581,024 to Meyer et al., having the same assignee as the present disclosure, addresses the somewhat related problem of correlating measurements made with different sensors on the same bottomhole assembly: due to a non-uniform rate of penetration, measurements made by different sensors take different amounts of time to pass through, for example, a formation having an identifiable thickness. As noted in Meyer, an important prerequisite is downhole depth correlation and vertical resolution matching of all sensor responses. U.S. Pat. No. 6,344,746 to Chunduru et al., having the same assignee as the present disclosure, addresses the problem of joint inversion of time-lapse measurements in which measurements are made at widely spaced intervals using sensors with different resolution. All of these problems could be avoided if accurate estimations could be made of the actual depth of the downhole assembly. See, for example, U.S. Pat. No. 6,769,497 to Dubinsky et al., and U.S. Pat. No. 7,142,985 to Edwards, both having the same assignee as the present disclosure. In the present disclosure, a method of determining depth shifts due to changes in drillstring length using downhole measurements is discussed.